For the application that I'm working on now, I need somehow to change the app image in background mode when displaying all of the open applications on a mobile device(to hide some sensitive data). I'm looking for a way to detect when an android app goes to the background and come back to foreground.

Before spending time to wrap that functionality with Java FFI capabilities I want to ask if somebody does this or know any way to do this without externals?

on mouseUp // or android equivalent, I don't code mobile...
if the focusedObject is not a card or control of this stack then // probably needs a repeat loop to test with...
objectToReturn = empty
end if
end mouseUp

In other words, if the object being focused on isn't part of your android program, doesn't that mean your program is in the background by default?

on openStack
mergNotify "UIApplicationWillResignActiveNotification"
mergNotify "UIApplicationDidBecomeActiveNotification"
end openStack
on UIApplicationWillResignActiveNotification pUserInfo
// Your code before the app goes to background
end UIApplicationWillResignActiveNotification
on UIApplicationDidBecomeActiveNotification pUserInfo
// Your code after the app comes back to foreground
end UIApplicationDidBecomeActiveNotification

on mouseUp // or android equivalent, I don't code mobile...
if the focusedObject is not a card or control of this stack then // probably needs a repeat loop to test with...
objectToReturn = empty
end if
end mouseUp

In other words, if the object being focused on isn't part of your android program, doesn't that mean your program is in the background by default?

Course, this could be faulty logic, just woke up

Unfortunately no. When an Android app goes into the background it is inaccessible. The user doesn't see it and can't interact with it until they re-open the app. However it does stay in RAM for a varying amount of time until the OS needs the memory, when it is summarily deleted. There is no notification when that happens. If the app is still in RAM but "closed" there are no messages sent when the user re-opens it because technically it is still open; that is, it doesn't relaunch, it just re-appears.

We could really use "suspend" and "resume" messages so the app knows when to take any necessary actions whenever the app comes to the front. If the OS has deleted the app from RAM then you do get all the usual startup/open-whatever messages. The problem is, when it is idling in the background and the user re-opens it, none of those occur.